In the TNG episode "Tapestry," Q is talking about the Picard that would never be if Picard had made a different choice in picking a fight with the Nausicaans. One of the events he mentions is the Picard who would “take charge of the Stargazer’s bridge when its captain was killed.” I know it isn’t mentioned again in the TV series, but is the story of that event detailed in any of the books or comics or other sources?

But it all started with one of the very first TNG novels, also by Michael Jan Friedman, Star Trek: The Next Generation Reunion - in which the remaining crew of the U.S.S. Stargazer reunite to see a former officer installed as ruler of the Daa'Vit Empire.

The reunion turns deadly when an assassin begins eliminating Stargazer
crew members one by one. Picard's present and former shipmates must
join forces to solve the mystery of the assassin who threatens the
peace of the entire galaxy.

There are also several other novels and comics that tie-into Picard's past: